1. Field of the Invention
The present invention relates to a process for converting synthesis gas to hydrocarbons, and more particularly to a process using a catalyst comprising cobalt and rhenium on an alumina support.
2. Description of the Prior Art
The reaction to convert carbon monoxide and hydrogen mixtures (defined herein as synthesis gas or syngas) to higher hydrocarbons over metallic catalysts has been known since the turn of the century. This reaction is commonly referred to as the Fischer-Tropsch or F-T synthesis. During World War II, Germany exploited a process employing the F-T synthesis for the production of gasoline and other hydrocarbon products. By 1944 a total of nine F-T plants was operating in Germany. The German process used primarily a catalyst composed of cobalt, magnesium oxide, thorium oxide and kieselguhr, in the relative proportions of 100:5:8:200. Later, most of the thoria was replaced by magnesia, primarily for economic reasons. Currently, commercial Fischer-Tropsch plants are operating in South Africa. These plants use a process employing a precipitated iron-based catalyst which contains various promoters to improve the stability and product distribution.
The common F-T catalysts are nickel, cobalt and iron. Nickel was probably the first substance to be recognized as capable of catalyzing the reaction of syngas to hydrocarbons, producing mainly methane (see, for example, "The Fischer-Tropsch Synthesis" by R. B. Anderson, Academic Press (1984), p. 2). Iron and cobalt are able to produce longer chain length hydrocarbons and are thus preferred as catalysts for the production of liquid hydrocarbons. However, other metals are also capable of catalyzing the F-T synthesis. Ruthenium is a very active catalyst for the formation of hydrocarbons from syngas. Its activity at low temperatures is higher than that of iron, cobalt or nickel; and it produces a high proportion of heavy hydrocarbons. At high pressures, it produces a high proportion of high molecular weight wax. Osmium has been found to be moderately active, while platinum, palladium and iridium exhibit low activities (see Pichler, "Advances in Catalysis", vol. IV, Academic Press, N.Y., 1952). Other metals which are active, such as rhodium, yield high percentages of oxygenated materials (Ichikawa, Chemtech, 6, 74 (1982)). Other metals that have been investigated include rhenium, molybdenum and chromium, but these exhibit very low activities with most of the product being methane.
Various combinations of metals can also be used for hydrocarbon synthesis. Doping cobalt catalysts with nickel causes an increase in methane production during F-T synthesis (see "Catalysis", vol. IV, Reinhold Publishing Co., (1956), p. 29). In U.S. Pat. No. 4,088,671 to T. P. Kobylinski, entitled "Conversion of Synthesis Gas Using a Cobalt-Ruthenium Catalyst", the addition of small amounts of ruthenium to cobalt is shown to result in an active F-T synthesis catalyst with a low selectivity to methane. Thus, these references teach that the combination of two or more metals can result in an active F-T catalyst. In general, the catalysts of these teachings have activities and selectivities which are within the ranges of the individual components.
Combinations of metals with certain oxide supports have also been reported to result in an improved hydrocarbon yield during F-T synthesis, probably due to an increase in the surface area of the active metal. The use of titania to support cobalt or cobalt-thoria is taught in U.S. Pat. No. 4,595,703, entitled "Hydrocarbons from Synthesis Gas". In this case the support serves to increase the activity of the metal(s) toward hydrocarbon formation. In fact, titania belongs to a class of metal oxides known to exhibit strong metal-support interactions and, as such, has been reported to give improved F-T activity for a number of metals(see, for example, S. J. Tauster et al, Science, 211, 1121 (1981)). Combinations of titania and two or more metals have also been shown to yield improved F-T activity. In U.S. Pat. No. 4,568,663, entitled "Cobalt Catalysts In the Conversion of Methanol to Hydrocarbons and for Fischer-Tropsch Synthesis", combinations of cobalt, rhenium and thoria and cobalt and rhenium supported on titania are claimed useful for the production of hydrocarbons from methanol or synthesis gas. This patent also indicates that similar improvements in activity can be obtained when cobalt-rhenium or cobalt-rhenium-thoria is compounded with other inorganic oxides. However, titania is the only support specifically discussed. The typical improvement in activity gained by promotion of cobalt metal supported on titania with rhenium is less than a factor of 2. We have found that the addition of rhenium to cobalt metal supported on a number of other common supports results in similar improvements in activity.
The only other examples in the literature of catalysts involving mixtures of cobalt and rhenium refer to completely different chemical reactions. For example, in Soviet Union Pat. No. 610558, a catalyst composed of cobalt and rhenium supported on alumina is taught to result in improved performance for the steam reforming of hydrocarbons. Steam reforming of hydrocarbons is a process completely different from hydrocarbon production via F-T synthesis and is believed to proceed by a completely different mechanism. Although some steam reforming catalysts can convert synthesis gas to hydrocarbons, such catalysts are not selective for the production of high carbon-number hydrocarbons (C.sub.3 and above) during conversion of synthesis gas. In fact, most commonly used steam reforming catalysts contain nickel as their active metal, and nickel produces mostly methane when used for syngas conversion.